This technique is well known in the art. A suitable tool comprises generally two separate tool parts which co-operate for producing said joint. A first tool part has the form of a punch which in a linear movement is driven in the direction of a co-axial second tool part in the form of a die with a die cavity at the bottom of which an anvil is arranged.
For making the joint the sheet formed members are positioned against the second tool part, the die, provided with movable die elements arranged sliding laterally on a support surface against the forces from a spring element. The spring element is generally constituted by a ring made of an elastomer or a toroid formed metal spring surrounding the movable die elements.
The approaching punch impacts on the surface of one of the members to be joined. The material of the two members is first drawn into the die cavity and subsequently due to the interaction between the punch and the anvil at the bottom of the cavity laterally extruded thereby displacing the movable die elements outwardly creating in the sheet formed members a mushroom formed button which interlocks the members.
U.S. Pat. No. 5,946,782, issued 7 Sep. 1999 discloses a conventional tool for making joints between sheet-formed member. FIGS. 14A and B, Prior Art, show a tool for making joints between sheet-formed members (4, 5) comprising a first tool-part (1, 2, 3) with a punch (1) and a side pressing element (sleeve) (2) and a second tool-part (6,11) provided with a support surface (9) from which an anvil (8) erects, a matrix comprising at least two matrix-parts (6) each having an upper active matrix surface and being arranged around said anvil (8). The matrix parts are surrounding an anvil (8) having a generally flat top surface (16). Each matrix part comprises a portion arranged sliding against said support surface (9) and being applied against the lateral surface of said anvil (8) by means of elastic means (7). Retention means (10) are arranged limiting the longitudinal movement of the matrix parts during the retraction of the punch (1) by engaging surface (12). The upper active matrix surface, the portion arranged sliding against said support surface (9) and said retention means (10) are arranged in that order from the top of the second tool-part (6, 11). A generally flat horizontal surface (15) of the punch will be brought into contact with the upper sheet formed member (4) when the punch is approaching the matrix-anvil part of the tool. The sheet formed members rest on the top surface of four identical matrix parts (6) forming the matrix. These matrix parts are surrounding an anvil (8) having a generally flat top surface. The matrix parts are held together by means of a resilient means (7) arranged in a grove on the surface of the matrix parts between the top surface and the inner sliding surface on the respective matrix part. A generally flat and horizontal co-operating sliding surface (9) is arranged on the anvil body (11) forming the top surface of a ring-formed element on said body. Side wall elements (19) are linking the upper part of the matrix element with an inner flange at the lower part of the element. This flange is extending inwardly, laterally around the lower part of said ring formed element thereby gripping around the same and preventing the dislocation of the matrix during the returning movement of the punch.
One of the problems with this type of it joint is to achieve sufficient strength against both shear forces and peeling forces. A number of tools exist which are more or less efficient when it comes to producing acceptable joints.